Method and apparatus for information retrieval from an optically readable storage medium

ABSTRACT

A method and apparatus for selectably retrieving information from any of a plurality of spirally-formed information tracks formed in a video disc by means of controlling a carriage for translating the video disc in a forward or a reverse direction relative to radiant beam information recovery means in a video disc player. Each information track is identified by a unique address and means are provided for selecting the address of a particular track to be retrieved. A prescribed sequence of drive signals is applied to a carriage motor dependent on the distance to be traveled by the carriage to retrieve the selected information, the drive signals being successively stepped downward to intermittently redetermine carriage motor speed as predetermined location thresholds are reached during carriage translation.

This is a continuation of application Ser. No. 295,629. filed Aug. 24,1981, which is a continuation of application Ser. No. 920,777, filedJune 30, 1978, now abandoned.

BACKGROUND OF THE INVENTION

This invention relates generally to information retrieval from anoptically readable storage medium and, more specifically, to a new andimproved method and apparatus for retrieving information from videodiscs and the like in a rapid and reliable manner.

Video disc players and the like are known for recovering informationthat has been stored, for instance, as a succession of light reflectiveand non-reflective regions along spirally-formed information tracks in adisc-shaped record carrier. The record carrier can be rotated at arelatively high rate, while an optical system is employed for directinga radiant beam, such as a laser beam, to impinge upon the informationtracks and for gathering a reflected beam that has been modulated by thereflective and non-reflective regions of the information track. Such aplayer includes a carriage for translating the video disc relative tothe radiant beam at a rate equal to the pitch of the spirally-recordedtracks and beam steering means for manipulating the radiant beam toprecisely follow the path defined by such tracks.

A frequency modulated electrical signal is recovered from the reflected,light modulated beam and is applied to appropriate signal processingcircuitry for deriving a video signal for display on a video monitor.Various control signals that are utilized to operate the player are alsoderived from the reflected beam.

Heretofore, the capability has existed of displaying a selected frame ofvideo information by prescribing the address of the frame or informationtrack wherein it is stored, and causing the carriage to translate in arapid manner to the vicinity of such information track. The addressinformation has been specially encoded in both of the vertical blankingintervals present in each revolution of an information track comprisingthe two standard fields of a video frame.

More particularly, in order to retrieve the selected frame orinformation track, the carriage has been translated at a uniform rate,greater than the normal play speed of the player, towards suchinformation track. During the course of translating the carriage, trackaddresses of at least some of the tracks crossed were detected, althoughit is obviously not possible to detect every track address in such amode of operation. As soon as it was detected that the selectedinformation track had been reached or crossed, the drive signal to thecarriage was terminated. Invariably, however, the carriage travelledpast the location of the selected track and it was at least necessary todrive the carriage once in the opposite direction to return to it.

By this prior technique, it was determined to always approach theselected information track from one direction when preparing to actuallystop the carriage at the track. In other words, if the carriage weredriven initially in the normal forward direction for retrieval, afterovershooting the selected track the carriage would be driven in thereverse direction past the selected track again. Upon passing the track,the reverse drive signal would be terminated and the carriage wouldagain overshoot. Finally, the normal play mode of the video disc playerwould be utilized to simply play into and stop at the selectedinformation track.

If the selected track was reached in such a search mode by rapidlytranslating the carriage in the reverse direction, the carriage drivesignal would be terminated upon reaching or passing the selected track,with the accompanying overshoot. From this position the video discplayer utilized the normal play mode to play into and stop at theselected track, since the carriage was then located on the proper sidefor approaching the selected track.

It will be apparent that the aforedescribed techniques for recoveringthe information stored in selected tracks were prone to deficiencies.For instance, overshoot when approaching the selected track from eitherthe forward or the reverse direction resulted in delays in retrievingthe information stored on the track. The requirement of alwaysapproaching the selected track from one direction compounded the delay.

Hence, there has been a need for an improved, rapid informationretrieval technique for use with the video disc players and the like,wherein information is optically recovered by means of a radiant beambeing impinged upon an information storage medium, that solves theaforedescribed problems. The present invention fulfills this need.

SUMMARY OF THE INVENTION

Briefly, and in general terms, the present invention provides a new andimproved method and apparatus capable of rapidly retrieving informationstored in a selected one of a plurality of optically readableinformation tracks by an information recovery system employing radiantbeam information recovery means.

Basically, the present invention includes an improved electronic methodand apparatus for controlling movement of an information storage medium,such as a disc-shaped record carrier, relative to a radiant beam that isutilized to recover information by scanning along information tracksformed therein. A particular information track may be targeted forrecovery in a retrieval mode of operation, for instance by specifying anaddress included with the information in the track. The location of thistarget track relative to the location of the information track currentlybeing scanned by the radiant beam is monitored and, as a function of thedifference in locations, signals are prescribed for controlling relativemovement between the target track and the radiant beam in order torapidly re-position the storage medium relative to the radiant beam toenable scanning of the targeted information track. More specifically,the rate of relative movement is progressively decreased in a prescribedmanner, from an initial translation rate determined by the distanceseparating the radiant beam and the targeted information track. Themanner of decreasing the rate of relative movement is selected tointermittently redetermine the rate at predetermined intervals as theradiant beam approaches the target track, and to ensure that the rate issuch that the radiant beam will not be caused to overshoot. Theseparating distance between the target track and the radiant beam may bedetermined by deriving addresses stored in the information tracks as theradiant beam scans in the retrieval mode and, by comparing theseaddresses with the address of the target track.

In a presently preferred embodiment, by way of example and notnecessarily by way of limitation, a video disc player embodying featuresof the present invention might include a carriage controller forcontrolling the position of a carriage on which a video disc is mountedfor translation relative to a beam of radiation employed for scanninginformation tracks on the video disc. A comparison of the address of aninformation track selected for retrieval with the address theinformation track being scanned by the beam of radiation is made and adetermination is reached whether the carriage should be driven in aforward or in a reverse direction to retrieve the target track. Beamsteering means for manipulating the radiant beam to precisely follow thepath defined by the tracks in a play mode of operation may be disabledin the retrieval mode.

As a function of the distance separating the beam of radiation and thetarget track, the carriage controller prescribes a sequence of drivesignals to be applied to a carriage motor for varying the rate ofmovement of the carriage. For example, the carriage controller mayprescribe one of four drive signals, resulting in a particular initialrate of movement, upon determining that the beam of radiation isseparated from the target track by at least a particular thresholddistance. As the carriage is moved at this initial rate, the separatingdistance between the beam of radiation and the target track decreases,and another distance threshold may be crossed. In such event, thecarriage controller switches a different drive signal of lower magnitudeto the carriage motor to effect a slowing of the carriage. Progressiveswitching of drive signals of lower magnitude is effected upon crossingeach prescribed distance threshold until the target track is reached andall drive signals are switched off, thereby stopping the carriage at thetarget track.

In a further aspect of the present invention, the drive signal appliedto the carriage motor immediately prior to stopping at the target trackis selected to be the normal play speed of the video disc player.Preferably, switching of this drive signal to the carriage motor alsore-enables the beam steering means so that the radiant beam accuratelyfollows the information, and therefore reliably recovers trackaddresses, as the carriage plays into the target track in the play modeand stops.

In an alternative embodiment of the present invention, monitoring of theseparating distance between the target track and the radiant beam isaccomplished during an initial portion of the retrieval mode by countingthe number of tracks crossed by the radiant beam. Such a technique maybe preferably when the carriage is being translated at such a high rateas to make detection of track addresses relatively unreliable. Due toeccentricities inherent in the information tracks, and the need for beamsteering means to accurately follow the tracks, monitoring of trackaddresses is again relied upon as the carriage shifts into the normalplay mode prior to stopping at the target track.

The method and apparatus for information retrieval of the presentinvention satisfies a need for rapid and reliable information retrievalfrom optically readable tracks by information recovery systems utilizingradiant beam information recovery means.

The above and other objects and advantages of this invention will beapparent from the following more detailed description when taken inconjunction with the accompanying drawings of illustrative embodiments.

DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a generalized block diagram of a video disc player in whichsome of the basic concepts of the present invention are illustrated;

FIG. 2 is a flow chart illustrating an alogorithm applicable to thecarriage controller shown in FIG. 1, for varying the carriage speed anddirection of movement in the search mode;

FIG. 3 is a waveform showing the response of the carriage motor as it isdriven to a target track in either the forward or the reverse direction;

FIG. 4 is an electrical schematic of one embodiment of a carriage driveras shown in FIG. 1 suitable for utilizing the results of the algorithmof FIG. 2;

FIG. 5 is a generalized diagram of a signal recovery subsystem, suitablefor use with the video disc player of FIG. 1; and

FIG. 6a is a fragmentary cross-sectional view of three informationtracks of a video disc, while FIG. 6b is a waveform applicable to thesignal recovery subsystem of FIG. 5.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1 of the drawings for purposes of illustration,there is shown a new and improved system for information retrievalembodying features of the present invention. In the drawings, the samenumeral will be used in the several views to represent the same element.

In FIG. 1, a generalized video disc player is shown for purposes ofillustrating the principles of the invention and incldues a spindle onwhich a video disc 10 is mounted for rotation by a spindle motor 12 at apreselected angular rate. Information is recorded on the video disc 10in a frequency modulated format as successive light reflective andnon-reflective regions along tracks that are formed either as acontinuous spiral or in discrete concentric rings. The video disc playeralso includes an optical system 14 which produces a laser read beam 16and directs it through an optical read head 18 having an objective lensfor focusing the read beam to a precise spot on the video disc 10.

A reflected beam 20, modulated by the information recorded in thetracks, is gathered by the read head 18 and returned through the opticalsystem 14 to a signal recovery subsystem 22. The spindle motor 12 ismounted on a carriage 24 for translation of the video disc 10 in thedirection indicated by a double-headed arrow 26 by means of a carriagemotor 28. Coarse steering of the read beam 16 along the informationtracks is accomplished by translating the carriage 24 either at auniform rate equal to the pitch of spirally-formed tracks or stepwise ifthe tracks are formed as discrete concentric rings.

A tracking subsystem 30 is included in the video disc player to enablethe read beam to follow eccentricities that are inherent in theinformation tracks with present technology. Also, since the descriptionthat follows will assume that the information tracks are formed in aspiral manner, wherein one complete revolution of the video disccomprises one information track, a stop motion subsystem 32 is shown inFIG. 1 to enable the video disc player to stop or freeze on a particulartrack. Both the tracking and stop motion subsystems receive a controlsignal on line 34 from the signal recovery subsystem 22.

The tracking and stop motion subsystems will be described below only asnecessary for a full understanding of the present invention. Specificdetails of these subsystems, as well as further details of certain othersubsystems shown in FIG. 1, are disclosed in greater detail in U.S. Pat.No. 4,332,022 entitled "Tracking System and Method for Video DiscPlayer", inventors Ceshkovsky et al, and assigned to the assignee of thepresent application. Reference is also made to U.S. Pat. No. 3,944,727entitled "Video Disc Player with Movable Mirror For Directing LightBeams Onto Reflective Disc," inventor James E. Elliott, and alsoassigned to the assignee of the present application. The disclosures ofU.S. Pat. No. 4,332,022 and U.S. Pat. No. 3,944,727 are herebyincorporated by reference.

The tracking subsystem 30 is employed for maintaining radial tracking ofthe focused read beam 16 on one information track, and is responsive tothe control signal on line 34 to develop an error signal on line 31 tothe optical system 14, indicating the offset from the preferredcenter-of-track position to the actual position. This tracking errorsignal is employed for controlling the movement of a radial trackingmirror in the optical system 14 to bring the light spot back onto thecenter-of-track position.

The tracking subsystem 30 normally operates in a closed loop mode ofoperation when the player is operating at a play speed. However, thetracking subsystem 30 is disabled in a retrieval mode, such that thedifferential tracking error is temporarily removed from controlling theoperation of the radial tracking mirror. The tracking subsystem 30 canalso be temporarily disabled by the stop motion subsystem 32, which thengenerates various combinations of signals on a line 33 to control themovement of the radial tracking mirror for directing the point ofimpingement of the focused spot from the preferred center of trackposition on a first track to a center of track position on an adjacenttrack in order to effect stop motion.

The signal recovery subsystem 22 develops an FM signal that includes thevideo information and all other information stored in the informationtracks, and applies that signal on a line 36 to a signal processingsubsystem 38. The latter subsystem includes a conventional FM detectorfor demodulating the FM signal into a standard format video signal,which is then applied on a line 40 to a video monitor 42 for display andto the stop motion subsystem 32. Means are also included in the signalprocessing subsystem 38 for separating the vertical sync signal from thevideo signal so that the sync signal can be applied on a line 44 to thestop motion subsystem 32 to be used in a manner described below.

Each information track or frame of video information that is recorded onthe video disc 10 is identified by a unique address encoded once in eachof the pair of vertical intervals between the two fields comprising aframe. For purposes of the present invention, the video signal from thesignal processing subsystem 38 is also applied on a line 40 to anaddress recovery subsystem 46, wherein the address associated with eachinformation track is decoded in any manner familiar to those of ordinaryskill in the art. These addresses, for instance may be encoded in aselected digital format on a selected horizontal line in each verticalinterval.

A signal representing the address information is then directed on a line48 to a function generator 50 and to a carriage controller 52. Thefunction generator 50, which may be embodied in a remote control,displays the address of the information track currently being read, andincludes means for selecting both a retrieval mode of operation and theaddress of the information track targeted for retrieval, in accordancewith the principles of the present invention.

In essence, when the retrieval mode is selected, the function generator50 applies both an enabling signal on a line 54 and the address of thetarget track on a line 56 to the carriage controller 52, wherein acomparison of the address of the current track being read with theaddress of the target track is made and a determination is reachedwhether the carriage 24 should be driven in a forward or reversedirection to retrieve the target track. Also, the carriage controller 52prescribes a sequence of drive signals to be applied to a carriagedriver 60 in order to move the carriage motor 28, and hence the carriage24 and the video disc 10, to retrieve the target track in a rapid mannerwithout overshooting it. The algorithm by which these determinations aremade is illustrated in FIG. 2, and will be described in detail below.The function generator 50 also applies a signal on a line 58 to thetracking subsystem 30 to disable it in the search mode as describedabove.

The determination of the direction in which the carriage 24 should bedriven to reach the target track results in either a forward (FWD)signal being applied from the carriage controller 52 to carriage driver60 over line 62 or a reverse signal (REV) being applied to the carriagedriver on a line 64. In addition, the sequence of signals prescribed bythe carriage controller 52, so that the carriage 24 rapidly homes in onthe target track, results in one of four carriage drive signals (S1, S2,S3 or S4) being applied to the carriage driver 60 from the carriagecontroller on one of the lines 66, 68, 70 or 72, respectively. Theoutput of the carriage driver 60 is directed over a line 74 to drive thecarriage motor 28, and a tachometer 76 is shown as being mechanicallyinterconnected to the carriage motor to provide an indication of itsactual speed and direction by means of a feedback signal on a line 78 tothe carriage driver.

The carriage drive signals S1-S4 represent four possible speeds at whichthe carriage motor 28 can be driven and hence four rates at which thecarriage 24 and the video disc 10 can be translated relative to the readbeam 16. One of these drive signals, S4, corresponds to the normal playspeed of the video disc player which results in translation of thecarriage 24 at a rate equal to the recorded pitch of the informationtracks. The other drive signals S3, S2 and S1 correspond to successivelygreater carriage translation rates.

In accordance with the present invention, the carriage controller 52prescribes a preferred sequence for applying the drive signals to thecarriage motor 28, as a function of the distance betwen the trackcurrently being read by the player and the track targeted for retrievalin the function generator 50, for retrieval of the information stored inthe target track. In this regard, as the carriage moves and the distanceto the target track decreases, the drive signal from the carriage driver60 on line 74 to the carriage motor 28 is sequentially stepped downwardas a sequence of distance thresholds, D1, D2 and D3 are crossed. Thisresults in a prescribed deceleration of the carriage motor 28 and hencethe carriage 24, as the target track is approached (see FIG. 3). Whenthe target track is within a prescribed distance, represented bydistance threshold D3, the tracking subsystem 30 is re-enabled by asignal on line 72 from the carriage controller 52. Then, as the targettrack is reached, all drive signals to the carriage driver 60 are set tozero, and the stop motion subsystem 32 is enabled by a signal on line 79from the carriage controller 52.

Referring now to FIG. 2, the manner in which the carriage controller 52determines the direction in which to drive the carriage 24 to retrievethe target track and prescribes an optimum sequence of drive signals isdiagrammed. It will be apparent that, in addition to the possibility ofimplementing the algorithm by means of suitable hardware, such asdigital logic elements, all or part of the algorithm may be performed byconventional programming on a digital computer or a microprocessor.

Initiation of a search mode by the function generator 50 commences witha conventional start step 80, followed by a step 82 in which thequestion is asked whether the address A2 of the target track (targetaddress) is greater than the address A1 of the track currently beingread by the video disc player (current address). An affirmative answerto this question indicates that the proper direction to drive thecarriage 24 to retrieve the target track is forward and leads to step84, resulting in the forward signal on line 62 being set to a returnstate (FWD=1), while the reverse signal on line 64 is set to a falsesignal (Rev=0). If the answer is no, then the forward signal on line 62and the reverse signal on line 64 are set to the false (FWD=0) and thetrue (REV=1) states, respectively, by step 86.

Once the direction that the carriage is to be driven has been fixedeither by step 84 or by step 86, a sequence of steps are utilized todetermine the distance D to be traversed to the target track, asrepresented by the absolute magnitude of the difference between thetarget address A2 and the current address A1.

More specifically, with reference to FIG. 2, the question is asked atstep 88 whether the distance D is greater than the first distancethreshold D1 from the target track. If the answer is yes, then the drivesignal S1 on line 66 is met to a true state (S1=1) in step 90, resultingin the carriage motor 28 being driven at a particular speed. Since thedistance threshold D1 represents the greatest distance to the targettrack, the drive signal S1 is selected to cause the carriage motor 28 tooperate at its fastest available speed until the next distance thresholdD2 is reached. On the other hand, if the answer to the question posed instep 88 is no, then the carriage motor 28 will be driven at a prescribedspeed less than the maximum in order that the carriage motor not bedriven at such a rate that it could not be stopped without overshootingthe target track. Initially the answer to the question posed in step 88may be no, of course, if the current track being read when the searchmode commences is closer to the target track than the distance thresholdD1.

As mentioned previously, eccentricities in the video disc 10, which areunavoidable with present technology, require utilization of a trackingsubsystem 30 such as that shown in FIG. 1 to accomplish fine steering ofthe read beam to accurately follow the path of the information tracks ina play mode of operation. It was also noted, however that in a retrievalmode of operation as described herein, the tracking subsystem 30 isdisabled. Notwithstanding this disablement of the tracking subsystem 30,a certain amount of FM information is recovered from the video disc 10and provided to the signal recovery subsystem 22 as the read beam 16rapidly crosses tracks in the search mode.

Although the FM information recovered from the disc is such that thevideo monitor 42 cannot provide a stable display, sufficient informationwill be recovered at various intervals to derive the addresses of sometracks as they are crossed. In this regard, identical addressinformation is diametrically encoded for each track, in the verticalinterval associated with each field comprising a frame, so that therewill be an opportunity to recover address information every halfrevolution of the disc. If the video disc is rotating at a typical 1800r.p.m., this will occur once approximately every 16 milliseconds. It iscontemplated, therefore, that the alorithm of FIG. 2 be repeated uponeach updating of the current address A1, typically at intervals of 16msec. Thus, after step 90, as well as all other steps wherein a drivesignal is set, the algorithm returns to the start step 80 inanticipation of updated information respecting the current address A1.

When the answer to step 88 is no, either because the target track isinitially closer to the current track being read than the distancethreshold D1, or because the carriage 24 has been driven closer in thesearch mode at the maximum speed represented by the drive signal S1, thequestion is posed by step 92 whether the distance D is greater than thenext distance threshold D2. If the answer is yes, then the drive signalS1 on line 66 is set to a false state (S1=0) and the drive signal S2 online 68 is set to a true state (S2=1) by step 94. Consequently, thecarriage motor 28 will be driven at a speed corresponding to the drivesignal S2, and the carriage controller will remain in this state untilthe next distance threshold D3 is reached.

From the foregoing, it should now be apparent that a primary purpose ofsequentially down stepping the drive signal applied to the carriagedriver 60 is to decelerate the carriage motor 28 in a relativelypredictable fashion and to intermittently redetermine the speed of themotor as the carriage approaches the target track. In this manner, theeffects of variability in the dynamic characteristics of particularcarriage motors and carriages is minimized by selecting the drivesignals and distance thresholds to allow intermittent redetermination ofcarriage position and speed during the course of homing in on the targettrack. As a result, the carriage is driven towards the target as rapidlyas practicable without overexciting the carriage motor and possiblyexperiencing overshoot.

To complete the algorithm, the question is posed at step 96 whether thedistance D is greater than the third distance threshold D3 to the targettrack and, if the answer is yes, the preceding drive signals S1 and S2are set to a false state (S1=0, S2=0), and the drive signal S3 that isapplied on line 70 from the carriage controller 52 to the carriagedriver 60 is set to a true state (S3=1) by step 98. When the distance Dfinally becomes less than the third distance threshold D3, but is stillgreater than zero, as determined by the next question posed in step 100,the fourth drive signal S4 is set to a true state (S4=1), and allprevious drive signals S1, S2 and S3 are set to a false state (S1=0,S2=0, S3=0) by step 102.

It will be recalled that in the presently preferred embodiment, thecarriage motor speed represented by the drive signal S4 is chosen to bethe normal play speed of the video disc player in which the carriage 24is translated at a rate equal to the pitch of the spiral tracks formedon the video disc 10. Therefore, the drive signal S4 is also applied online 72 to the tracking subsystem 30 in order to re-enable it uponre-establishing play speed, since the tracking subsystem 30 was disabledby a signal on line 58 from the function generator 50 upon initiation ofthe search mode. This was because the various sequential drive signalsS1, S2 and S3 all cause the carriage 24 to translate at rates greaterthan the normal play speed represented by the drive signal S4. At thesehigher translation rates, it is neither practical nor desirable toattempt to fine steer the read beam 16 as tracks are rapidly crossed. Ofcourse, when the drive signal S4 is applied, the tracking subsystem 30preferably is re-enabled so that the read beam 16 will most accuratelyfollow the information tracks and track addresses can be reliablyretrieved.

Finally, when the difference D becomes zero, making the answer to thequestion posed in step 100 yes, all previous drive signals S1, S2, S3and S4 are set to a false state (S1=0, S2=0, S3=0 and S4=0) by step 104.Of course, with all the drive signals S1-S4 set to a false state, thecarriage 24 will stop. At the same time, a stop signal on a line 79 tothe stop motion subsystem 32 is set to a true state (STOP=1) by step104. The purpose of this stop signal is enable the stop motion subsystem32 such that the target track will be frozen on the display of the videomonitor 42. Since the information tracks are recorded in spiral fashion,it is required that read beam 16 be jumped back once each revolution ofthe video disc 10 so that the read beam repeatedly retraces the sameframe of video information. A particular manner of generating anappropriate jump-back signal and controlling the tracking subsystem isdescribed in detail in the aforementioned copending related applicationof Dakin et al, entitled "Video Disc Player".

Briefly, the stop motion subsystem 32 is employed as a means forgenerating a plurality of control signals for application to thetracking subsystem 30 on line 33 to achieve the movement of the focusedspot tracking the center of a first information track to a separate andspaced location in which the spot begins tracking the center of the nextadjacent information track. The stop motion subsystem 32 performs itsfunction by detecting a predetermined signal recovered from thefrequency modulated video signal which indicates the proper positionwithin the recovered frequency modulated video signal for initating thejumping operation. This detection function is achieved, in part, byinternally generating a gating circuit conditioned by the vertical syncsignal received on line 44 to indicate that portion of the recoveredvideo signal received on line 40, within which the predetermined signalshould be located.

In response to the predetermined signal, which has been termed a "whiteflag" in the aforementioned related application, the stop motionsubsystem 32 generates a first control signal for application to thetracking subsystem 30 for temporarily interrupting the application ofthe differential tracking error to the radial tracking mirrors in theoptical system 14. The stop motion subsystem 32 generates a secondcontrol signal for application to the radial tracking mirrors forcausing the radial tracking mirrors to leave the center of trackingposition on a first information track and jump to an adjacentinformation track. The stop motion subsystem terminates the secondcontrol signal prior to the focused spot reaching the center of focusposition on the next adjacent information track.

A third control signal may be generated by the stop motion subsystem 32at a time spaced from the termination of the second control pulse. Thethird control pulse is applied directly to the radial tracking mirrorsfor compensating for the effects on it which were added by the secondcontrol pulse. While the second control pulse is necessary to have thereading beam jump from a first information track to an adjacentinformation track, the spaces involved are so small that the jumpingoperation cannot always reliably be achieved using the second controlsignal alone. Therefore, the third control signal may be employed forcompensating for the effects of the second control jump pulse on theradial tracking mirror at a point in time when it is assured that thefocus spot has, in fact, left the first information track and has yet tobe properly positioned in the center of the next adjacent informationtrack. Finally, the differential error signal may be gated through tothe radial tracking mirror at a time calculated for the gated portion ofthe differential tracking error to assist the compensation pulse inbringing the focused spot under control upon the center of trackposition of the next adjacent information track.

Referring now specifically to FIG. 3, control of the carriage motordrive signal, and hence carriage motor speed, as a function to thedistance D to the target track, is illustrated by waveforms, includingapproaches from both the forward and the reverse directions.

Assuming for the moment that the carriage 24 is positioned further fromthe target track than the distance threshold D3 and should be driven inthe forward direction to reach the target track, it can be seen that themaximum drive signal S1 is initially applied to the carriage driver 60so that the carriage motor 28 will assume its maximum available speed.As the carriage 24 approaches the distance threshold D3, the drivesignal applied to the carriage driver 60 steps down to S2. Of course,the carriage motor 28 and the carriage 24 have a certain amount ofinertia and the speed decays to a speed dictated by the drive signal S2over a period of time. Preferably, the dynamic characteristics of thecarriage motor 28 and the carriage 24 are critically damped so that aspeed represented by the drive signal S2 is reached as quickly aspossible.

It should be noted that, in any event, the dynamic characteristics ofthe carriage motor 28 and carriage 24, as well as the distancethresholds such as D3 and D2, should be chosen so that the carriagemotor speed does decay to the speed represented by the drive signal S2prior to the carriage reaching the next distance threshold D2.Otherwise, the purpose of stepping down the drive signal to the carriagedriver in order to redetermine the carriage motor speed at particularintervals along route to the target track will be defeated. Thoseskilled in the art will recognize that particular carriage motors andcarriages will display a certain amount of variability in their dynamicresponse characteristics and that the various drive signals and distancethresholds should be selected with this variability in mind, i.e.,sufficient distance between thresholds should be allowed so that even acarriage motor and assembly with a relatively slow time response willdecelerate to the speed represented by the next drive signal prior tothe next distance threshold being reached.

This process of stepping down the drive signal and allowing the speed ofthe carriage motor 28 to substantially completely decay before againstepping down the drive signal at the next distance threshold isrepeated until the normal play speed represented by drive signal S4 isreached and the tracking subsystem 30 is re-enabled. Then as thecarriage 24 reaches the target track, the carriage 24 is stopped and thestop motion subsystem 32 is enabled as described above.

For purposes of convenience, the identical sequence of drive signals anddistance thresholds is shown for implementation when the target trackmust be approached in the reverse direction. For particular systems, itmay be that the carriage motor and the carrier will display differingresponse characteristics in the forward and the reverse directions, inwhich case a different sequence of drive signals and distance thresholdswould be selected. It will also be appreciated that the particularnumber of drive signals and distance thresholds in the sequence in notcritical to the present invention.

A particular electrical circuit for implementing the carriage driver isshown in FIG. 4. The circuit includes a first quad analog switch 108having four IN, OUT and CONTROL connections. The drive signals S1-S4 areapplied individually to the four CONTROL connections on the lines 66,68, 70 and 72, while the four IN connections are tied to a positivesupply voltage V on a line 110. The four OUT connections are eachconnected individually through a resistor R1-R4, respectively, to thefirst two IN connections of a second quad analog switch 112 on a line114. The second switch 112 receives its corresponding two CONTROL inputsindividually from the forward and reverse signals on lines 62, 64 fromthe carriage controller 52. Only two connections are utilized on thesecond analog switch 112. A suitable commercial device for these twoquad analog switches is a Motorola type MC14016.

The first OUT connection of the second analog switch 112 is appliedthrough a resistor R5 on line 116 to the inverting input of a firstoperational amplifier 118 having a feedback resistor R6. The second OUTconnection of the second analog switch 112 is likewise applied throughan equivalent resistor R5 on line 120 to the inverting input of a secondoperational amplifier 122, having an indentical feedback resistor R6.The output of the second operational amplifier 122 is then applied tothe inverting input of a third operational amplifier 124 through aresistor R7 on a line 128, the operational amplifier having an identicalfeedback resistor R7 to provide unity gain.

The operation of this circuit as thus far described will now beexplained. Depending on which, if any, of the drive signals S1-S4 is ina true state, the corresponding OUT connection of the first analogswitch will have the supply voltage V applied to it. Thus, a currentwill be supplied through a particular resistor, such as resistor R1 whendriver signal S1 is set to a true state (S1=1), to the two INconnections on the second analog switch 112 on line 114. This currentwill be directed through either one of the two OUT connections of thesecond switch 112 depending on which of its two control signals, theforward or reverse signals from the carriage controller 52, are in atrue state. Hence, the current will cause an inverted voltage to appearat the output of the first operational amplifier 118 if the forwardsignal is true (FWD=1). On the other hand, a noninverted voltage willappear at the output of the third operational amplifier 124 if thereverse signal is true (REV=1).

One of these two voltages are then applied through identical resistorsR8 on lines 130 or 132 for summing with the feedback signal from thecarriage tachometer 76 through a resistor R9 on line 78, at theinverting input of a fourth operational amplifier 134, having a feedbackresistor R10. The output of this fourth operational amplifier 134 isthen applied to a power amplifier 136 on a line 138 for appropriatelyenergizing the carriage motor 28 over line 74 in the forward or thereverse direction. In accordance with well-known principles, thecarriage tachometer 76 is intended to generate an equal and oppositesignal to the drive signal from the first operational amplifier 118 orthe third operational amplifier 124, as the case may be, to null theinput at the fourth operational amplifier 134, i.e., negative feedbackcontrol.

An alternative approach for determining carriage location or distance Dfrom the target track can be described with reference to FIGS. 5 and 6.

A suitable subsystem for implementing the signal recovery subsystemshown in FIG. 1, is disclosed in FIG. 5. A diode detector array 140includes a central photodetector 142 for deriving the informationalcontent of the modulated light beam and has a pair of diametricallyopposed tracking diodes 144, 146 on either side. An electrical signalproportional to the intensity of light received on the central detector142 is provided on lines 148, 150 to a summing junction and then to awide band amplifier 152, having an output directed on line 36 to thesignal processing subsystem 38 as described above. Each tracking diode144, 146 is disposed to detect the portion of the modulated light beamcorresponding to individual tracking spots, which are produced bysplitting the read beam 16 into three separate beams by means of adiffraction grating in the optical system 14, and each diode generatesan electrical signal on lines 154 and 156, respectively, to trackingpreamplifiers 158, 160. One preamplifier 158 has an output directed on aline 162 to the inverting input of an amplifier 164 and the output ofthe other preamplifier 160 is directed on a line 166 to the noninvertinginput of the amplifier. The output of the amplifier 164 then provides atracking error signal on line 34 to the tracking subsystem 30 and to thestop motion subsystem 32 as described in connection with FIG. 1.

A fragmentary radial cross section of three tracks of the video disc areillustrated in FIG. 6A; while in FIG. 6B, the open loop differentialtracking error signal is illustrated which appears on line 34 at theoutput of differential amplifier 164 when the tracking subsystem 30 isdisabled and the carriage translates rapidly in a search mode ofoperation.

It will be apparent that the waveform of FIG. 6B can be utilized as anindicator of track crossings and to provide a count of the number oftracks crossed in the search mode. Considering this information incombination with the target address A2 and the address of the particulartrack from which the search is initiated. A continuous monitoring ofcarriage position relative to the target track could be derived bycounting track crossings. It should be noted, however, that such atechnique would be preferable only at relatively high speeds oftranslation of the carriage. This is because the eccentricities inherentin a video disc tend to create false "track crossings" even when thecarriage is standing still, if the tracking subsystem is disabled. Athigh rates of translation of the carriage, the effects of these false"track crossings" due to eccentricities will not be significant, but asthe speed of the carriage slows towards a play speed their effects mayprevent an accurate track count. Hence, in an alternative embodiment, itis contemplated that track crossings may be counted by means of the openloop differential tracking error signal at relatively high rates ofspeed, while dependence will be shifted to detection of addresses as thecarriage speed approaches play speed.

The aforedescribed information retrieval system of the present inventionsatisfies a need for improved systems capable of rapidly accessinginformation tracks in apparatus of the type utilizing a radiant beam tooptically read the information stored in such tracks.

It will be apparent from the foregoing that, while particular forms ofthe invention have been illustrated and described, various modificationscan be made without departing from the spirit and scope of theinvention. Accordingly, it is not intended that the invention belimited, except as by the appended claims.

We claim:
 1. In an apparatus for selectively retrieving information froma plurality of optically readable information tracks formed in aninformation storage medium, wherein the information includes locationidentification data identifying the location of different segments ofsaid information, said apparatus further including means for impingingan incident beam of radiation upon the storage medium, and informationrecovery means for recovering information from a modulated beam ofradiation produced thereby, the combination comprising:carriage meansfor moving the storage medium and the incident beam of radiationrelative to one another to cause the incident beam to move across saidtracks in a search mode of operation and in a direction toward apredetermined position on said storage medium; carriage driver means forproducing a drive signal to be applied to said carriage means to effectsaid relative movement of said incident beam in said direction in saidsearch mode of operation, said means for recovering informationincluding means for detecting location identification data from therecovered information, and intermittently redetermining the incidentbeam position, from location identification data in the informationrecorded, during said relative movement of said incident beam acrosssaid tracks in said search mode and in said direction toward saidpredetermined position; and control means, operating after said incidentbeam has been moved to within a prescribed distance from saidpredetermined position, for controlling said carriage driver means toalter said drive signal and thereby cause said carriage means toprogressively slow and then cease its search mode movement when saidincident beam is at said predetermined position.
 2. The apparatus asclaimed in claim 1, including:means for varying the speed of movement ofsaid incident beam across said tracks in said search mode of operation;and means, responsive to said means for detecting locationidentification data from the recoverd information, for intermittentlyredetermining and subsequently changing said speed of movement as saidincident beam approaches said predetermined position in said searchmode.
 3. In an apparatus for selectively retrieving information from aplurality of optically readable information tracks formed in aninformation storage medium, wherein the information includes locationidentification data identifying the location of different segments ofsaid information, said apparatus further including means for impingingan incident beam of radiation upon the storage medium and informationrecovery means for recovery information from a modulated beam ofradiation produced thereby, the combination comprising:carriage meansfor moving the storage medium and the incident beam of radiationrelative to one another to cause the incident beam to move across saidtracks in a search mode of operation and in a direction toward adesirable information segment on said storage medium; carriage drivermeans for producing a drive signal to be applied to said carriage meansto effect said relative movement of said incident beam in saiddirection, in said search mode of operation, toward said desirableinformation segment to be recovered by said recovery means, said meansfor recovering information including means for detecting locationidentification data from the recovered information and intermittentlyredetermining the incident beam position, from location identificationdata in the information recorded, during said relative movement of saidincident beam toward said desirable information segment in said searchmode and in said direction toward said desirable information segment;and control means, responsive to said information recovering meansdetecting the location identification data for controlling said carriagedriver means to alter said drive signal and thereby cause said carriagemeans to progressively slow and then cease its search mode movement whensaid incident beam is at said desirable information segment.
 4. Theapparatus as claimed in claim 3, including:means for varying the speedof movement of said incident beam across said tracks in said search modeof operation; and means, responsive to said means for detecting locationidentification data from the recovered information, for intermittentlyredetermining and subsequently changing said speed of movement as saidincident beam approaches said desirable information segment in saidsearch mode.
 5. In an apparatus for selectively retrieving informationfrom a plurality of optically readable information tracks formed in aninformation, storage medium wherein the information includes locationidentification data identifying the location of different segments ofsaid information, said apparatus further including means for impingingan incident beam of radiation upon the storage medium, and informationrecovery means of recovering information from a modulated beam ofradiation produced thereby, the combination comprising:carriage meansfor moving the storage medium and the incident beam of radiationrelative to one another to cause the incident beam to move across saidtracks in a search mode of operation and in a direction toward apredetermined position on said storage medium; carriage driver means forproducing a drive signal to be applied to said carriage means to effectsaid relative movement of said incident beam in said direction in saidsearch mode of operation, said means for recovering informationincluding means for detecting location identification data from therecovered information, and intermittently redetermining the incidentbeam position, from location identification data in the informationrecorded, during and relative movement of said incident beam across saidtracks in said search mode and in said direction toward said position;and control means for controlling said carriage driver means to altersaid drive signal and thereby cause said carriage means to cease itssearch mode movement when said incident beam is at said predeterminedposition without reversing the direction of said relative movement. 6.The apparatus as claimed in claim 5, including:means for varying thespeed of movement of said incident beam across said tracks in saidsearch mode of operation; and means, responsive to said means fordetecting location identification data from the recovered information,for intermittently redetermining and subsequently changing said speed ofmovement as said incident beam approaches said predetermined position insaid search mode.
 7. In an apparatus for selectively retrievinginformation from a plurality of optically readable information tracksformed in an information storage medium, wherein the informationincludes location identification data identifying the location ofdifferent segments of said information, said apparatus further includingmeans for impinging an incident beam of radiation upon the storagemedium, and information recovery means for recovering information from amodulated beam of radiation produced thereby, the combinationcomprising:carriage means for moving the storage medium and the incidentbeam of radiation relative to one another to cause the incident beam tomove across said tracks in a search mode of operation and in a directiontoward a desirable information segment on said storage medium;.[.carriage driver means for producing a drive signal to be applied tosaid carriage means to effect said relative movement of said incidentbeam in said direction, in said search mode of operation toward saiddesirable information segment to be recovered by said recovery means,.].said means for recovering information including means for detectinglocation identification data from the recovered information andintermittently redetermining the incident beam position, from locationidentification data in the information recorded, during said .[.relativemovement of said incident beam toward said desirable information segmentin said.]. search mode .[.and in said direction toward said desirableinformation segment.].; and control means, responsive to saidinformation recovering means detecting the location identification datafor .[.controlling said carriage driver means to alter said drive signaland thereby cause.]. .Iadd.causing .Iaddend.said carriage means to ceaseits search mode movement .[.when.]. .Iadd.in response to .Iaddend.saidincident beam .[.is at.]. .Iadd.reaching .Iaddend.said desirableinformation segment without reversing the direction of .[.saidrelative.]. movement .Iadd.of said carriage means.Iaddend..